1. Field of the Invention
This invention relates to a nitride semiconductor device, and more particularly to a nitride semiconductor device having the structure of a heterojunction field effect transistor.
2. Background Art
Circuits such as switching power supplies and inverters are based on power semiconductor devices including switching devices and diodes, which are required to have such characteristics as high breakdown voltage and low on-resistance (RON). There is a tradeoff relation between the breakdown voltage and the on-resistance (RON), which relation depends on the device material. With the progress of technology development, the on-resistance (RON) of power semiconductor devices is reduced to nearly the limit for silicon (Si), which has been the main device material. For further reduction of on-resistance (RON), the device material needs to be changed. For example, wide bandgap semiconductors such as gallium nitride (GaN), aluminum gallium nitride (AlGaN), and other nitride semiconductors and silicon carbide (SiC) can be used as switching device materials to improve the tradeoff relation determined by the material, thereby dramatically reducing on-resistance (RON).
On the other hand, nitride semiconductors such as GaN and AlGaN can be used for heterojunction field effect transistors (HFETs) based on the AlGaN/GaN heterostructure. HFETs can achieve low on-resistance through the high mobility of the heterointerface channel and the high electron concentration due to piezopolarization caused by heterointerface strain.
Such a nitride semiconductor device can be made on a substrate such as sapphire (Al2O3) or SiC. However, the sapphire substrate has poor heat dissipation because of its large thermal resistance. On the other hand, while the SiC substrate is superior in heat dissipation, it has a high manufacturing cost, and it is technically difficult to fabricate a large-diameter substrate. In light of these circumstances, it is desirable from a comprehensive viewpoint to use a silicon (Si) substrate, which is relatively superior in heat dissipation, low-cost, and capable of achieving a large-diameter wafer.
However, Si and the AlGaN/GaN heterostructure are greatly different in lattice constant. For this reason, strain-induced cracks are likely to occur, and the thickness of a GaN layer that can be crystal grown without cracks is limited to about 1 to 2 micrometers. The maximum breakdown voltage of a GaN-HFET is determined by the thickness of the GaN layer. Typically, when a GaN-HFET device is formed on a conductive substrate, a voltage is applied between the drain electrode and the substrate. Therefore the device breakdown voltage strongly depends on the film thickness of the GaN layer. Because the critical electric field of GaN is about 3.3 MV/cm, the maximum device breakdown voltage is 330 volts when the film thickness of the GaN layer is 1 micrometer. For example, a film thickness of 2 micrometers or more is needed for obtaining a breakdown voltage of 600 volts or more.
On the other hand, some conventional techniques have been proposed for obtaining a high-quality GaN film free from cracks and the like.
For example, JP2001-230410A discloses a technique for obtaining a high-quality GaN film by using selective lateral growth to form a GaN crystal in the region where the electric field is concentrated during operation.
An article titled “AlGaN—GaN HEMTs on Patterned Silicon (111) Substrate”, IEEE Electron Device Letters, Vol. 26, No. 3, March 2005, discloses a technique for growing a crack-free GaN film on a silicon substrate by providing rectangular ridges thereon.
However, even when these techniques are used, it is extremely difficult to obtain a high-quality GaN film free from defects and cracks, where the GaN film has a film thickness of several micrometers or more for particular use in power semiconductor devices.
Thus, in order to achieve a GaN-HFET having a high breakdown voltage of 600 V or more formed on a Si substrate, it is urgent to develop a technique for forming a crack-free, good GaN film of several micrometers or more. Moreover, this is also important for high-frequency GaN devices as well as power semiconductor devices, because a thick GaN layer is needed to avoid the deterioration of operating speed due to parasite capacitance between the electrode and the substrate when a Si or other conductive substrate is used.